Washing household appliances such as dishwashers and/or washing machines comprise a water circulation pump unit for recirculating water during the washing processes, and a drainage pump unit for removing or draining water from the appliance during a drainage process, which normally rotates at a determined speed of rotation and for a determined period of time.
Drainage pump units comprise an impeller and, generally speaking, a permanent-magnet synchronous motor for rotating the impeller at a determined speed of rotation, by means of which drainage is caused, as disclosed for example in patent documents EP 287984 B1 and ES 2162544 B1. The synchronous motors are connected to the mains supply, which comprises a determined mains frequency, the speed of rotation of the impeller depending on said mains frequency, which is normally substantially constant. The drainage flow depends on the speed of rotation of the impeller, and therefore on the frequency applied to the synchronous motor.
Washing household appliances can also comprise control means for controlling the supply of the drainage pump unit, which enable said units to be switched on at a determined point in time and switched off after a determined time period, when the programme cycle has ended or when deemed convenient. This can be set beforehand in accordance with the washing programme used for example, and the control means have the function of switching on the pump unit to start a drainage process during which the drainage pump unit causes the removal of the water, and of switching off said unit to end said drainage process. Consequently, the drainage processes result in the drainage pump unit being switched on for a preset time period at the predetermined frequency, without taking into account parameters that may optimise or improve the efficiency of said processes or which may even improve the reliability of the drainage pump units. This may cause a premature fault in the drainage pump unit, and due to the tendency of concentrating components in washing appliances, it may become increasingly difficult to replace said unit.
Washing household appliances that overcome this drawback by making the pump unit bigger are known, although this may result in an excessive cost that cannot always be assumed. Some washing household appliances comprise control means to overcome these disadvantages, preventing the need for an increase in size, said control means being adapted to control the supply of the drainage pump unit not only for starting and ending the drainage process, but also for controlling said supply during said drainage process.
Document EP1942219A1 discloses, for example, a washing household appliance of this type. The control means comprised in said appliance can cause the drainage pump unit to switch on and/or off during the drainage process. For this purpose, the household appliance comprises level sensors to detect the water level, and the control means can determine said water level in accordance with the detection of the level sensor. The control means thus cause, during the drainage process, the drainage pump unit to switch on at a determined frequency or said pump unit to switch off in accordance with said determined water level, thereby resulting in a more effective process and also increasing the reliability of the drainage pump unit as it is switched on for a smaller time period in each drainage process.
In some washing household appliances a drainage pump unit with a variable frequency is used, which comprises an impeller and a BLDC-type motor for rotating said impeller. Document EP1783264A2, for example, discloses an appliance that comprises a motor of this type, and also discloses a control method for improving the control of the drainage pump unit. The consumption current of the motor is determined, and said current is associated with a water level. The frequency of the motor supply voltage is modified in order to vary the drainage water flow, in accordance with the related water level. For this purpose the control means also compare the level associated with a preset level.
Document EP 2248935 A1 discloses a washing household appliance that comprises a drainage pump unit with an impeller and a motor supplied with an alternating supply voltage of a determined frequency from among a plurality of possible preset frequencies, and control means adapted to regulate the frequency of the motor supply voltage between the preset frequencies.